It seemed like small change at the time, and the dems were not all pro illegals back then.
I think you're looking back with rose colored glasses. I was an adult in 1986, and well remember the push for amnesty as being very much a big deal. The left put tremendous pressure on Reagan to sign it, all the while promising;
"This will be the first and last amnesty, and we'll build a border fence to stop future illegal immigration, if you'll just sign it."
Naturally, they were lying through their teeth. I felt so at the time, and Reagan should have too. Why he trusted those bastards, I'll never know.